1. Technical Field
The invention relates generally to vehicle drive systems, and more specifically to battery powered drive systems such as are used in battery powered electric vehicles or hybrid vehicles.
2. Discussion of Art
Recently, electric vehicles and hybrid electric vehicles have become increasingly popular. These vehicles are typically powered by one or more batteries, either alone or in combination with an internal combustion engine. In electric vehicles, the one or more batteries power the entire drive system, thereby eliminating the need for an internal combustion engine. Hybrid electric vehicles, on the other hand, include a small internal combustion engine to supplement the battery power, which greatly increases the fuel efficiency of the vehicle.
Traditionally, the electric and hybrid electric propulsion systems in these vehicles required the use of large batteries, ultracapacitors, flywheels, or a combination of these elements so as to provide sufficient energy to power the electric motor. While generally effective, the size and weight of the elements reduced the overall efficiency of the propulsion system and presented challenges for integration into the vehicles themselves.
Another challenge related to conventional electric propulsion systems was that the nominal voltage of the energy storage units (i.e., batteries and/or ultracapacitors) set the overall system voltage. Thus, the energy available to power the electric motor was limited to the energy available in the energy storage units themselves. Such a configuration limited the overall reliability and efficiency of the electric propulsion system, as the voltage demands of the electric motor were often far greater than the energy storage unit voltage. To combat this issue, several alternative electric propulsion system configurations have been devised. In particular, U.S. Pat. No. 5,373,195 shows the use of a bi-directional boost converter to decouple the energy storage unit voltage from a direct current (DC) link voltage, wherein the DC link is coupled to the electric motor. The bi-directional boost converter acts to increase, or “boost”, the voltage provided from the energy storage unit to the DC link to meet the power demands of the electric motor. In fact, the ratio of the DC link voltage to the energy storage unit voltage is typically greater than two-to-one. The bi-directional boost converter enables such an increase in voltage supplied to the DC link without the need for an increase in the size of the energy storage unit or units.
While the bi-directional boost converter successfully allows for an increased supply of voltage to the DC link without a corresponding increase in size of the energy storage unit(s), the efficiency of the bi-directional boost converter degrades during certain operating modes. In particular, during high-speed and high-power acceleration and deceleration of the vehicle, the ratio of DC link voltage to battery voltage is often greater than 2.5:1. Under these operating modes, the level of electrical current to which the components of the boost converter are subjected is very high, and therefore there is a subsequent need for proper thermal design to dissipate heat in the power electronic components of the boost converter. This thermal cycling stress on the components of the bi-directional boost converter poses a potential reliability issue, as well as a reduction in overall system efficiency.
Furthermore, during high-speed and high-power deceleration, a concept known as “regenerative braking” enables power at potentially relatively high voltage generated by the electric motor to be cycled back through the bi-directional boost converter for storage in the energy storage unit(s). However, at high DC link voltage to battery voltage ratios, there are again high losses within the bi-directional boost converter that require proper heat dissipation in the electrical components. Also, the regeneration power provided to the energy storage unit is often limited by the charge acceptance of the energy storage unit itself, which further reduces the efficiency of the system.
Therefore, it is desirable to provide an electric and/or hybrid electric propulsion system having greater overall system efficiency along with an increased level of energy capture during high-power regenerative braking.